Metal coated synthetic diamonds embedded in a synthetic resinous matrix bond

ABSTRACT

A diamond abrasive device, such as a grinding wheel, comprising metal clad diamond particles embedded in a bonding matrix of resinous material, which is characterized in that the diamond particles comprise MD particles.

United States Patent [1 1 [111 3,902,873

Hughes Sept. 2, 1975 [54] METAL COATED SYNTHETIC DIAMONDS 3,276,852 10/1966 Lemelson 51/298 EMBEDDED IN A SYNTHETIC RESINOUS 3,356,473 12/1967 Hull et al 51/295 3,385,684 5/1968 Voter 51/298 MATRIX BOND 3,528,788 9/1970 Seal 51/295 [75] Inventor: Frank Hallmark Hughes,

Johannesburg, South Africa FOREIGN PATENTS OR APPLICATIONS [73] Assignee: Industrial Distributors (194 1,142,688 9/1957 France 51/295 Limited, Johannesburg, South Africa OTHER PUBLICATIONS [22] Filed: Dec. 28, 1970 Armored Diamond Wheels, L. I. Smith et al., May pp No: 102,039 1966, pp. 24-27 and 36.

Related US. Application Data Continuation of Ser. No. 761,171, Sept. 20, 1968, abandoned.

GRIND/1V6 [F /027m) (a m 770) Primary -Examiner-Donald J. Arnold Attorney, Agent, or F irmYoung & Thompson A diamond abrasive device, such as a grinding wheel, comprising metal clad diamond particles embedded in a bonding matrix of resinous material, which is characterized in that the diamond particles comprise MD particles.

ABSTRACT 7 Claims, 12 Drawing Figures mA 5-: BLEND Nb/l Mc BL END WHEEL NUMBER PAIENTEUSE i S 3,902,873

sum 6 o g Jan! METAL'COATED SYNTHETIC DIAMONDS EMBEDDED IN A SYNTHETIC RESINOUS MATRIX BOND This application is a continuation of my application Ser. No. 761,171, filed Sept. 20, 1968, now abandoned.

This invention relates to diamond abrasive grinding devices and, in particular, to diamond abrasive grinding wheels.

It is well known and indeed logical that the diamond, as the hardest and most wear-resistant material known, is extremely suitable for grinding hard and abrasive materials, such as tungsten carbide, glass, corundum and the like. It is, therefore, not surprising that'production engineerslook to a diamond abrasive grinding wheel as one of the most suitable tools for grinding such hard and abrasive materials. However, hitherto it was the accepted view that diamond is not suitable for the grinding of soft metals, particularly mild steel, and that the use of diamond abrasive devices is expensive.

Up to the present time, the art is familiar with the use of two principal types of diamond particles in abrasive devices. These types are generally known as resin bond diamond (RD) and metal diamond (MD). A third type, SD, refers to diamond used in saw applications. The RD, MD and SD classes of particles may be obtained today by both natural and synthetic means. The RD class of particles is typical of the particles employed in a bonding matrix of resinous material while the MD particles are used in metal bonds. In order to appreciate the nature of the present invention, it is necessary to outline the characteristics of these two classes of particles. The SD particles may also be used with great advantage in abrasive articles in metal bond form although in the art at the present time they are looked upon as being relatively expensive for this purpose. However, the Applicant envisages their use in the arrangement of the invention and for this reason specifies that the term MD is used herein to include SD particles.

MD particles have a higher impact resistance than RD particles which are friable and these two classes of particles lie within different, well-defined ranges of impact resistance. An RD particle is of an irregular nature and it exhibits the ability to splinter during the course of grinding operations before drilling of the cutting formation develops. In consequence it is continually generating new cutting formations so that a wheel tipped with RD particles is particularly suitable for grinding operations. MD particles, on the other hand, generally present a blocky crystal, but irregularly shaped MD crystals also exist. An important characteristic of all MD particles is that they do not tend to splinter and irrespective of whether they are of blocky or irregular shape, their grinding formations dull before any fracturing is likely to arise.

Until recent times RD particles had decided disadvantages. In normal operations where, say, the RD particles were embedded in a resin matrix of suitable quality, the splintering took place in such a manner that it became possible for whole particle bodies to loosen in the matrix and to be knocked out of the matrix by impact on the surface being ground. This is an aspect which hindered the use of resin-bonded wheels. The cause of the difiiculty lay in the fact that once a splinter of a particle had broken away, a void might be developed into the heart of the particle into which the peripheral regions of the particle might be urged. This 2 contributed to the particle freeing itself from the anchorage in'the resin.

The problem outlined above-was met by the novel concept of cladding RD diamond particles with a suitable metal coating. The metal-coated RD particles were then embedded in a bonding matrix of resinous material and the overall performance of such wheels was increased substantially. In particular, the tendency for particles to be pulled out of the bonding matrix as the particles splintered was materially reduced. An upto-date statement on this development is to be found in British Pat. No. 1,154,598. The action of the metal cladding is to hold the particle rigidly as a unit despite the development of fracture lines and the splitting off I of splinters which is characteristic of RD diamonds.

As far as the Applicant is aware, MD particles have never been used in a resin bond but only in a metal bond.

the bond, thus ruining the wheel surface as an economically useful unit for this purpose. In terms of money, the cost of removing a cubic centimeter of soft steel in the En 8/9 range with a resin bond RD wheel is about $1.28 as compared with 0.60 cents for an aluminum oxide wheel. This disparity is lessened fairly appreciably by metal cladding the RD particles but, in any event, the RD metal cald wheel is not looked upon as having any useful application in the grinding of soft metals. In fact, to date no diamond grit has been looked upon as useful for this purpose.

It is an object of the present invention to provide a general purpose diamond abrasive grinding device which can grind a wide range of materials with practically acceptable efficiency and economy, and which is particularly suitable for grinding soft metals.

According to the present invention, the Applicant has found that quite unexpectedly and contrary to the accepted view that diamond particles in general are not suitable for the grinding of soft metals, a substantial advance in diamond abrasive grinding of soft metals is achieved by substituting metal clad MD particles for RD particles in a bonding matrix of resinous material.

More particularly, according to one aspect of the invention, a diamond abrasive device incorporating metal clad diamond particles embedded in a bonding matrix of resinous material is characterized in that the diamond particles comprise MD particles.

According to another aspect of the invention, a method of grinding a workpiece, preferably a workpiece with a hardness lying in the range from VPN to VPN 265, comprises the step of subjecting the workpiece to abrasion by metal clad MD diamond particles embedded in a bonding matrix of resinous material.

In reaching the present invention the Applicant investigated the grinding of various materials with resinbonded diamond abrasive grinding wheels and during the course of the investigation, carried out a series of tests on soft steels in the En 9 range using metal clad RD particles and unclad RD particles. The results indicated that the metal clad RD particles were indeed capable of being used for cutting of such soft metals, but

not to an extent that held out any high hopes for their general use in this field. Applicant also explored the position using metal clad MD grits in a resin bond and this part of the research was carried out with no high expectations in the light of past knowledge. Even the most modern literature does not envisage the possibility of these particles having any worthwhile use in the grinding of soft metals, and the language of the above-identified British patent supported this belief in its reference to particles splintering in use. MD particles do not splinter, while the effectiveness of RD in grinding operations is dependent mainly upon the particle breaking to present new grinding surfaces.

However, the tests unexpectedly showed that metal clad MD particles embedded in a bonding matrix of resinous material were indeed of great value in the grinding of soft steels and that their use in this field opens up unsuspected possibilities. Further tests showed that metal clad MD particles in a resin bond are also eminently suitable for the grinding of soft metals other than soft steels, and can grind virtually any material with practically acceptable efficiency and economy. A general purpose diamond abrasive grinding device can therefore be envisaged.

The reasons for the unexpected results achieved with metal clad MD particles in a resin bond are not yet fully understood, but it is clear that there is more to it than the simple explanation that the metal coating permits better retention of the diamond particles in the resin bond.

The MD particles may be metal coated and embedded in a resin matrix in any suitable manner as will be clear to a man skilled in the art. The methods disclosed in the above British patent may, for example, be used. The metal cladding preferably comprises 55% by weight of the metal clad diamond particles and the preferred metal coating comprises nickel. The resin matrix preferably comprises a synthetic thermosetting binder resin and may include suitable filler material.

The size of the MD particles may lie in the range 70 to 120 mesh. Preferably, the MD particles comprise 100 mesh or 100/120 mesh grit.

A grinding device according to the invention may comprise a wheel in which the resin-bonded metal clad MD particles form a peripheral skin about a hub.

It appears that when grinding soft metals such as mild steel, the grinding efficiency increases with the rigidity of the hub material. It has been found that for a general purpose grinding wheel according to the invention suitable for grinding virtually any material, good results are obtained with a hub wholly or partially composed of aluminum, such as an aluminum phenolic hub, and with a hub comprising bakelite, such as a fiber-f1lled phenolic hub.

Other objects, features and advantages of the invention will become apparent from the following more particular description, taken in connection with the accompanying drawings, in which:

FIG. 1 is a graphical comparison of the grinding efficiencies of various types of diamond grit on similar material according to Example I.

FIG. 2 is a graphical comparison of the grinding efficiencies of two types of diamond grit on different materials according to Example II.

FIG. 3 is a graphical comparison of the grinding efficiencies of resin-bonded metal clad MD grit at different volumetric removal rates according to Example III.

FIG. 4 is a graphical representation of the relationship betwcen grinding table traverse speed and total wheel cost for different cross-feeds according to Example III.

FIG. 5 is a graphical comparison of the grinding efficiencies of different types of diamond grit on a combination of tungsten carbide and steel at different downfeeds according to Example IV.

FIG. 6 is a graphical comparison of the total wheel cost of the different types of diamond grits of FIG. 5 when grinding a combination of tungsten carbide and steel at different down-feeds according to Example IV.

FIG. 7 is a graphical comparison of the grinding efficiencies of different types of diamond grit on tungsten carbide alone at different down-feeds according to Example IV.

FIG. 8 is a graphical comparison of the total wheel cost of the different types of diamond grits of FIG. 7 when grinding tunsten carbide alone at different downfeeds according to Example IV.

FIG. 9 is a perspective view of a flat grinding wheel according to the invention.

ing wheel according to the invention.

FIG. 12 is a cross-sectional view of the wheel of FIG.

To illustrate the merits of the present invention, the.

following examples of tests conducted with resin-bond diamond abrasive grinding wheels are given:

EXAMPLE I Tests were conducted on similar mild steel workpieces with grinding wheels carrying different diamond abrasive grits to determine the performance of the various grits under identical conditions.

FIG. 10 is a cross-sectional view of the wheel of FIG.

FIG. 11 is a perspective view of a cup-shaped grind- Wheel Specification Diamond Types Metal Cladding I50 X mm. EN 9 (55 carbon) Diamond lOO (4.4 cts/cm Concentration Wheel Speed 35.6 meters/sec.

Grinding Machine Driving motor Cross feed Downfeed Table Traverse Rate Coolant Length of Tests Volumetric Removal Rate Jones & Shipman 540 l mm. per table transverse reversal 0.025 mm/pass l2 meters/min.

Bryto 5 (I'll solution) 3 hours 300 mm/min.

D1Al" is the US. industry standard from USAS B 74.1-1963 and identifies a particular resinoid bond wheel made by practically all wheel makers. RDA

means resinoid diamond abrasive, while *MDA" means metal-bond diamond abrasive. MC" means The surface finish, expressed as center line average in micro inches, is shown in Table III below.

TABLE III metal clad, while S refers to the high strength of the specially selected MD crystals.

The grinding efficiencies obtained with the various types of diamond grit, expressed as the G ratio which is the ratio of the volume of workpiece removed to the volume of wheel worn away, are shown in Table I below and are compared graphically in FIG. 1 of the attached drawings.

TABLE I Wheel No. Diamond G. Ratio 1 RDA-MC 430 2 MDA-MC-Blend 660 3 MDA-S-MC-Blend 1250 It will be seen that for the grinding of soft EN9 mild steels, considerably higher G ratios are obtained with metal clad MD particles than with metal clad RD particles.

EXAMPLE II Tests were conducted on different materials with grinding wheels carrying grits nos. l and 2 of Example I under the same conditions as that of Example I with the exception that:

a. the downfeed for all the materials except the EN9 mild steel was 0.01 mm/pass, the downfeed for the EN9 mild steel remaining at 0.025 mm/pass.

b. the volumetric removal rate for all the different materials except the BN9 mild steel was I mm /min., the removal rate for the EN9 mild steel remaining at EXAMPLE III The following tests were conducted on cast steel to investigate the performance of resin-bonded metal clad MD grit with high volumetric removal rates when grinding large areas such as that experienced on press plattens, which could be as large as 10 X 3 meters. A tolerance of 0.025 mm cannot be achieved over such a large area with conventional grinding wheels which require frequent dressing and adjustment.

Test Conditions Machine Magerle F l0 Surface Grinder Workpiece BS l5 I964 800 X200 mm. Wheel Type DIAI Peripheral Wheel on an aluminum phenolic huh: (I) 254 mm X 12.5mm X I27 mm (2) 254 mm X 25 mm X I27 mm Diamond Metal clad MD grit I00 mesh Resin Bond I00 concentration Metal Cladding 55% by weight Nickel Wheel Speed 27.4 meters/sec. Downfeed 0.02 mm per pass Crossfeed 3.0 mm per traverse reversal (2) 4.5 and 6.0 mm per traverse reversal I6, 23 and 30 meters/min. Bryto 5 (I()0:I) 9 liters/min.

Table Traverse Speed Coolant The results obtained are shown in Table IV below and the G ratios are compared graphically in FIG. 3 of the accompanying drawings. The curves designated as A, B and C in FIG. 3 represent the results designated as A, B and C respectively in Table IV.

300 mm lmin. The G ratios and the power drawn are TABLE IV shown in Table II below.

. T bl T' G d v I R I The G ratios obtained with the two grits m question are Jigs 6 23 fig'gi ljzfi compared graphically in FIG. 2 of the attached draw- A 12 5 mm wide whee 3 0 mm crossfeed ings. It is clear that for the softer metals, the MDA-MC- 416 Blend grit is far superior to the RDA-MC grit. Although 53 is: {.25 the performance of the MDA-MC-Blend IS inferior to 25 mm wide wheel 45 mm crossfeed that of the RDA-MC on tungsten carbide, it is still 16 360 1.42

55 23 288 2.12 within practically acceptable limits. 30 78 2.83

TABLE II workpiece workpiece G. Ratio Power Drawn Kwh No. Material RDA-MC MDA-MC- RDA-MC MDA-MC- Blend Blend A Cast Iron 384 I000 1.52 2.08 B Brass 332 785 2.04 2.37 C BN9 Mild Steel 430 660 2.72 2.99 D Copper 246 600 I .76 2.07 E Aluminum I76 250 I .68 1.5 3 F H.S.S. 98 H0 3.l3 3.15 G Tungsten Carbide 262 I03 5.09 4.40

' TABLE IN-continued Table Traverse Grinding Volume Removal Speed meters/min. Ratio G Rate cm/min.

costs, at $3.75 per hour and the cost of a diamond wheel at $25.00 per cubic centimeter of bond, which 8 the sum of time cost and diamond abrasive wheel cost is between 13 and 14 cents per cubic centimeter of steel removed. The results indicate that the use of resin-bonded metal clad MD grit grinding wheels for the grinding of soft metals is economical.

EXAMPLE IV The following tests were conducted to assess the performance of resin-bonded metal clad MD grit when grinding tungsten carbide on its own and when grinding a combination of tunsten carbide and steel shank material as is often encountered in industry.

TEST PARAMETERS:

Machine Wheel type/size Diamond Jones 8L Shipman Model 141 1 Semiautomatic surface grinder with 7.5 HP. Spindle Drive Motor. DlAl Peripheral Wheel on 21 Fiberfilled Phenolic Hub 175 mm X mm X 51 mm. Nickel clad RD grit 100/120 mesh Resin bond 100 Concentration (designated as RDA'MC) Nickel clad MD grit 100/120 mesh Resin bond 100 Concentration (designated as MDA-MC-Blend) Wheel speed Wheel peripheral speed Downfeed per pass Total downfeed per test C rossfeed Table traverse speed 2.720 R.P.M.

25.4 meters per second .01. .025 and .05 mm 1.5 mm per traverse reversal 16 meters per minute Workpiece Coolant Coolant flow rate No. of tests per Change are average costs applicable in'Germany, the results are shown in'Table V below: 1

Vard Harmet G6 (9471 WC. 671 C0) Airds EN9 Steel (55 Carbon) 150 X 100 mm arranged in alternate pieces of 12.7 mm TC 25.4 mm steel, 4 times. in grinding direction.

P20 tungsten carbide (75% WC,

15% Tit-mic. 10 71 0). 18 pieces with faces 5 mm X 12.7 mm arranged as a block 150 X 75 mm.

Water Bryto 5 100 l 9 liters per minute At least 3 Series 1 Series 2:

Taking time cost comprising machine, labor and overhead costs, at $5.00 per hour and wheel cost at TABLE V Table Traverse Time Cost Wheel Cost Total Cost Speed meters/min. cents/cm cents/cm" cents/cm" removed removed removed A. 12.5 mm wide wheel 30 mm crossfeed 16 8.75 6.00 14.75 23 6.50 6.50 13.00 30 5.75 8.25 14.00 B. 25 mm wide wheel 4.5 mm crossfeed 16 5.75 7.00 12.75 23 4.75 8.75 13.50 4.00 14.00 18.00 C. 25 mm wide wheel 6.0 mm crossfeed Table V indicates that wheel cost must not be considv ered in isolation but must be considered in conjunction with time cost. Since time cost decreases and wheel cost increases with table speed, there is an optimum table speed at which the total cost is a minimum. The relationship between table traverse speed and total cost for different crossfeeds is indicated graphically in FIG. 4 of the accompanying drawings. The curves designated A, B and C in FIG. 4 represent the results designated A, B and C respectively in Table V. It will be seen that at a table traverse speed of 23 meters per minute,

$20.00 per cm", which are average costs applicable in South Africa, the results obtained are shown in Tables VI and VII below.

It will be noticed that 3 downfeed increments were used, namely 0.01, 0.025 and 0.05 mm. The reason for this is that a downfeed of 0.01 mm is generally used in Europe, whereas a standard downfeed of 0.025 mm (0.001 inch) is used in Britain, and the USA. The 0.05 mm downfeed was investigated to determine the effect of an increase in downfeed and thus in production rate.

Series 1. Grinding a combination of TC and Steel Wheel Downfeed G ratio Removal per cm" No. mm Time Mins. Time Wheel Total Cost Cost Cost Cents Cents Cents l (RDA-MC) .01 186.2 12.20 102.4 10.4 112.8 .025 126.3 4.50 37.7 15.3 53.0 .05 36.7 2.96 25.0 52.5 77.5 2 (MDA-MC- Blend) .01 169.0 12.40 104.2 11.5 115.7 .025 201.0 4.20 3513 97 45.0 .05 63.3 2.78 23.4 30.5 53.9

TABLE VII Series 2. Grinding tungsten carbide alone Wheel Downfeed G ratio Removal per cm No. mm Time Mins. Time Wheel Total Cost Cost Cost Cents Cents Cents l. (RDA-MC) 2. (MDA-MC- Blend) Grinding efficiency (G ratio), which is often taken as the criterion as to whether a wheel is good or bad, is a misleading factor if the time taken on the grinding operation, the time cost and wheel cost for a given amount of material are not also taken into account. Grinding efficiency can only be considered as a dependable criterion if the wheels being compared are of the same size and cost the same, and if the tests are carried out under identical conditions.

FIG. 5 of the accompanying drawings compares the grinding efficiencies of Table V1 for the combination of tungsten carbide and steel. FIG. 6 compares the total cost figures of Table VI. The curves designated 1 and 2 in FIGS. 5 and 6 represent the results for wheels Nos. 1 and 2 respectively in Table VI.

The grinding of a combination of tungsten carbide and steel is generally regarded as one of the more awkward jobs in the grinding world. It is therefore significant that as shown in FIG. 5, the G ratio obtained with metal clad MD grit is better than that obtained with metal clad RD grit over most of the range tested. Also, the total cost is generally lower with MDA-MC-Blend grit than with RDA-MC grit, except at low downfeed values. At a downfeed of 0.025 mm (0.001 inch). the MDA-MC-Blend grit is significantly better, whereas at a downfeed of 0.01 the RDA-MC grit is slightly better. The G ratio is lower for both grits at a downfeed of 0.05 mm. but the higher production rate has the effect of lowering the cost.

The grinding efficiencies of Table VII for tungsten carbide alone are compared graphically in FIG. 7 of the accompanying drawings. FIG. 8 compares the total cost figures of Table VII. The curves designated 1 and 2 in FIGS. 7 and 8 represent the results for wheels Nos. 1 and 2 respectively in Table VII.

It is clear from FIGS. 7 and 8 that when tungsten carbide on its own is to be ground, the RDA-MC grit is superior to MDA-MC-Blend both from the point of view of efficiency and total cost.

Although resin-bonded metal clad MD, grit is less efficient than resin-bonded metal clad RD grit when tungsten carbide on its own is ground, the efficiency and economy of the metal clad MD grit is nevertheless of practically acceptable value. Considering the efficiencies and total cost obtained with metal clad MD grit when grinding a combination of tungsten carbide and steel and when grinding soft metals on their own, it is clear that a resin-bonded metal clad MD grit diamond abrasive grinding wheel can grind virtually any material efficiently and economically.

In consequence of the unexpected results achieved, the Applicant has determined that it is now possible to employ resin-bonded metal clad MD diamond abrasive wheels in a manner which could make them economic in the grinding of soft metals. It has previously been mentioned that in grinding a soft steel such as EN9 mild steel, aluminum oxide wheels may be used at a cost of about 0.60 cents per cubic centimeter of material removed. Resin-bonded metal clad MD grinding wheels in accordance with the invention appear to be able to do the same work for a figure in the region of 1.71 cents per cubic centimeter of material removed which, on first appearance, is not encouraging. However, by bringing the price down to 1.71 cents, it is possible to envisage a general purpose diamond abrasive grinding wheel which may be used for practically all purposes.

Instead of the operator having to replace the grinding wheel according to the material on which Work is to be performed, he may now use a wheel of the invention for all purposes. The time lost in replacing a conventional 1 1 diamond wheel with, say, an aluminum oxide wheel where a soft steel workpiece is to be treated subsequent to a hard metal workpiece can be avoided with a saving in time cost and the inconvenience of dressing the conventional abrasive wheel for the finishing cuts.

Another significant advantage of grinding wheels in accordance with the invention, as compared with a conventional aluminum oxide wheel, is that where high precision work is required to be done, the resin-bonded metal clad MD wheel can maintain accuracy within fine tolerances over extended working periods. Not only does this result in work of a superior quality, but the cost resulting from loss of wheel in dressing, dressing time and machine downtime for wheel replacement is minimized.

It will be appreciated that many variations are possible without departing from the scope of the appended claims. For example, the MD particles may be natural or synthetic diamond, but generally synthetic MD particles are preferred.

Any suitable metallic material may be used for cladding the MD particles. Apart from the preferred metal nickel, cobalt or any of the other metallic coating materials disclosed in the above British patent may be used, namely, platinum group metals, gold, silver, copper, molybdenum, titanium, aluminum, manganese, cadmium, tin, zinc, chromium, tungsten, iron, zirconium and alloys containing at least one of these metals.

The metal cladding may comprise to 70% by weight of the total weight of the composite metal clad MD particles. More particularly, the metal cladding may comprise 50 to 60% by weight, preferably 55%, of the composite metal clad MD particles.

Any suitable resin matrix may be used, such as that disclosed in the above British patent, namely, phenolic, epoxy, polyimide, alkyd, unsaturated polyester, silicone, polybenzinimidazole and polyamidimide.

As shown in FIGS. 9 and 10, a flat grinding wheel according to the invention comprises an aluminum phenolic hub 1 carrying an abrasive peripheral skin 2 of resin-bonded metal clad MD diamond particles. Peripheral skin 2 is bonded to hub 1 under heat and pressure in well-known manner.

Instead of an aluminum phenolic hub, the wheel may be provided with a hub comprising bakelite, such as fibet-filled phenolic hub.

Referring now to FIGS. 11 and 12, the wheel comprises a cup-shaped hub 3 of suitable material carrying an annular skin 4 of resin-bonded metal clad MD diamond particles round the outer periphery of the cup on lip 5. As in the case of the fiat wheel, annular skin 4 is bonded to lip 5 under heat and pressure in wellknown manner.

I claim:

v1. A diamond abrasive grinding wheel for grinding soft metal having a hardness lying in the range from VPN 95 to VPN 265, comprising a hub and an annular 12 abrasive zone around the periphery of the hub, the abrasive zone comprising a bonding matrix of resinous material selected from the group consisting of phenolic, epoxy, polyimide, alkyd, unsaturated polyester, silicone, polybenzinimidazole and polyamidimide having embedded therein synthetic diamond particles coated with a metal selected from the group consisting of nickel, cobalt, silver, copper, molybdenum, titanium, aluminum, manganese, cadmium, tin, zinc, chromium, the platinum group metals, gold, tungsten, iron, zirconium and alloys containing at least one of said metals, the synthetic diamond particles being characterized in that they comprise synthetic MD particles and the metal coating comprising 10 to by weight of the composite particles.

2. A diamond abrasive grinding wheel as claimed in claim 1, wherein the metal coating comprises approximately 55% by weight of the composite particles.

3. A diamond abrasive grinding wheel as claimed in claim 1, wherein the sizes of said synthetic MD particles lie in the range 70 to 120 mesh.

4. In a diamond abrasive grinding device for soft metal having a hardness lying in the range from VPN to VPN 265, an abrasive surface comprising metal coated synthetic MD particles embedded in a bonding matrix of resinous material selected from the group consisting of phenolic, epoxy, polyimide, alkyd, unsaturated polyester, silicone, polybenzinimidazole and polyamidimide resin, the metal coating comprising 10 to 70% by weight of the composite particles and comprising a metal selected from the group consisting of nickel, cobalt, silver, copper, molybdenum, titanium, aluminum, manganese, cadmium, tin, zinc, chromium, the platinum group metals, gold, tungsten, iron, zirconium and alloys containing at least one of said metals.

5. A diamond abrasive grinding device as claimed in claim 4, wherein said metal coating comprises approximately 55% by weight of the composite particles.

6. A diamond abrasive grinding device as claimed in claim 4, in which the sizes of said particles lie in the range 70 to mesh.

7. A method of grinding a workpiece having a hardness lying in the range from VPN 95 to VPN 265, comprising the step of subjecting the workpiece to abrasion by metal clad synthetic MD particles embedded in a bonding matrix of resinous material selected from the group consisting of phenolic, epoxy, polyimide, alkyd, unsaturated polyester, silicone, polybenzinimidazole and polyamidimide resin, the metal coating comprising 10 to 70% by weight of the composite particles and comprising a metal selected from the group consisting of nickel, cobalt, silver, copper, molybdenum, titanium, aluminum, manganese, cadmium, tin, zinc, chromium, the platinum group metals, gold, tungsten, iron, zirconium, and alloys containing at least one of said 

1. A DIAMOND ABRASIVE GRINDING WHEEL FOR GRINDING SOFT METAL HAVING A HARDNESS LYING IN THE RANGE FROM VPN 95 TO VPN 265, COMPRISING A HUB AND AN ANNULAR ABRASIVE ZONE AROUND THE PERIPHERY OF THE HUB, THE ABRASIVE ZONE COMPRISING A BONDING MATRIX OF RESINOUS MATERIAL SELECTED FROM THE GROUP CONSISTING OF PHENOLIC, EPOXY, POLYIMIDE, ALKYD, UNSATURATED POLYESTER, SILICONE, POLYBENZINIMIDAZOLE AN POLYAMIDIMIDE HAVING EMBEDDED THEREIN SYNTHETIC DIAMOND PARTICLES COATED WITH A METAL SELECTED FROM THE GROUP CONSISTING OF NICKEL,
 2. A diamond abrasive grinding wheel as claimed in claim 1, wherein the metal coating comprises approximately 55% by weight of the composite particles.
 3. A diamond abrasive grinding wheel as claimed in claim 1, wherein the sizes of said synthetic MD particles lie in the range 70 to 120 mesh.
 4. In a diamond abrasive grinding device for soft metal having a hardness lying in the range from VPN 95 to VPN 265, an abrasive surface comprising metal coated synthetic MD particles embedded in a bonding matrix of resinous material selected from the group consisting of phenolic, epoxy, polyimide, alkyd, unsaturated polyester, silicone, polybenzinimidazole and polyamidimide resin, the metal coating comprising 10 to 70% by weight of the composite particles and comprising a metal selected from the group consisting of nickel, cobalt, silver, copper, molybdenum, titanium, aluminum, manganese, cadmium, tin, zinc, chromium, the platinum group metals, gold, tungsten, iron, zirconium and alloys containing at least one of said metals.
 5. A diamond abrasive grinding device as claimed in claim 4, wherein said metal coating comprises approximately 55% by weight of the composite particles.
 6. A diamond abrasive grinding device as claimed in claim 4, in which the sizes of said particles lie in the range 70 to 120 mesh.
 7. A method of grinding a workpiece having a hardness lying in the range from VPN 95 to VPN 265, comprising the step of subjecting the workpiece to abrasion by metal clad synthetic MD particles embedded in a bonding matrix of resinous material selected from the group consisting of phenolic, epoxy, polyimide, alkyd, unsaturated polyester, silicone, polybenzinimidazole and polyamidimide resin, the metal coating comprising 10 to 70% by weight of the composite particles and comprising a metal selected from the group consisting of nickel, cobalt, silver, copper, molybdenum, titanium, aluminum, manganese, cadmium, tin, zinc, chromium, the platinum group metals, gold, tungsten, iron, zirconium, and alloys containing at least one of said metals. 